Average expression stability values (M) of seven candidate reference genes computed using geNorm. Ranking was performed for all RNA samples (TOTAL), and for the subsets +2.5H, +6H and RESISTANT. M-values of the remaining genes at each step during stepwise exclusion of the least stable gene are shown. The genes are ranked according to increasing expression stability (decreasing M-values) starting from the least stable gene on the left. The two most stable genes are on the right. The horizontal dotted line indicates the cutoff M-value (1.5).